This invention relates generally to improvements in high resolution optical coupling devices, and more particularly to such devices which employ arrays of solid state photosensitive elements, such as charge-coupled devices (CCDs).
Solid state cameras are available both for laboratory and commercial use. Such cameras also are used for x-ray imaging when the x-ray image is arranged to impinge on a phosphor screen. The phosphor screen produces a corresponding light image that is transferred to a light sensor by an optical system. In order to obtain the highest performance of the camera, it usually is desirable to have the light sensor optically coupled via a bonding agent to the last optical element of the optical system and to operate the light sensor below room temperature. (As used herein, the term "last optical element" refers to the optical element (such as a fiber optic bundle, lens, or image intensifier) that is coupled by the bonding agent to the light sensor.)
In the case where the solid state camera employs a CCD array, it generally is assumed that the shape of the light-receiving face of the CCD wafer is spherical, and the mating face of the last optical element is shaped to match the assumed spherical shape of the CCD wafer. The substrate and CCD wafer generally are attached via a bonding agent that cures or sets at a temperature elevated above room temperature. The substrate material of a CCD ordinarily has a thermal coefficient of expansion which is different from that of the CCD wafer. As the CCD/substrate unit is cooled to room temperature, the difference in the thermal coefficients of expansion cause the CCD/substrate assembly to become either convex or concave, as observed from the CCD wafer side thereof. Furthermore, cooling the CCD/substrate assembly to operate below room temperature causes the shape to become even more convex or more concave.
In a CCD camera wherein the CCD wafer is optically coupled to an optical system via an optical-coupling material such as an optical grade epoxy, cooling the CCD/substrate assembly leads to a tension type of stress in the edge regions of the optical-coupling material for a convex shape and in the central region of the optical-coupling element for a concave shape. The repeated stress cycles that result from cooling and warming as the camera system is turned on and off can lead to failure of the bonding of the optical-coupling material to the CCD and/or the mating element of the optical system.
FIG. 1 shows a prior art optical coupling device involving a fiber optic bundle 20 with a face shaped to match the assumed spherical shape of the CCD wafer 1 bonded to substrate 2. Optical-coupling material 4, which can be an optical grade epoxy such as EPO-TEK 301-2, optically couples CCD wafer 1 to the hopefully matching spherical shape of a fiber optic bundle 20, which is the mating "last optical element" of the optical system. Shaping the face of fiber optic bundle 20 leads to improved resolution. However, the light-receiving face of CCD wafer 1 typically is non-spherical. Thus, the optimum resolution of the camera system including the optical coupling device of FIG. 1 is not realized, since the non-sphericity of face of CCD wafer 1 is not accounted for in the optical coupling of the CCD wafer and the optical system.
Furthermore, many applications require high performance of the solid state camera system which can only be achieved by operating the CCD below room temperature. Since the optical coupling material itself has a thermal expansion coefficient typically one to two orders of magnitude larger than the thermal expansion coefficients of the CCD wafer/substrate material and fiber bundle or last optical element, such operation at low temperatures also generates considerable stress on the bonding at the optical-coupling material interfaces between the CCD wafer and the fiber optic bundle 20 or last optical element. Consequently, repeated thermal cycling often leads to failure of such interface bonding. Failure of the interface bonding of optical coupling material in turn causes a significant reduction the performance of the camera system and has a high probability of damaging the CCD wafer beyond repair.
Such failure can create serious economic loss as a result of downtime of the imaging instrument and the cost of replacement of an expensive CCD wafer. Furthermore, the medical profession cannot tolerate poor resolution in x-ray imaging.
Thus, there is an unmet need for a low cost structure and method for both (1) matching the shape of a CCD wafer to the shape of a fiber optic bundle 20 or other last optical element and (2) reducing or eliminating stress which occurs at the interfaces between the CCD wafer, last optical element, and the optical coupling material therebetween as a result of thermal cycling.